On the surface, this may sound crass, but many teachers are underpaid, and a little extra green in their stocking could make a big difference during the holiday season. When you absolutely don't know what to give, cash is a very versatile choice that no one regrets receiving. To personalize your gift, have your child write the card, or include a card containing cash in a batch of homemade muffins or cookies.
If you're still not comfortable with giving the gift of cash, consider donating money in the teacher's name to a charity of their choosing. The gift will have much more significance if it's a donation to the teacher's favorite charity and not yours. A brief conversation should yield some useful clues that will help you make a good choice. You can do this locally or on a grander scale and go global. Either way, it's a personal gift that will not only benefit you and the teacher, but the recipients as well.